1. Field of the Invention
The present invention relates to a method and apparatus for manufacturing an internal combustion spark plug.
2. Description of the Related Art
Various methods and apparatus for manufacturing spark plugs for internal combustion engines have heretofore been proposed (see, for example, Japanese Patent. Laid-Open No. 121143/1999 (page 10, FIG. 1) and EP 0540159 A1 (FIG. 4)).
3. Problems to be Solved by the Invention
In recent years, there has been a demand for spark plugs for internal combustion engines capable of igniting a fuel gas at a proper timing under any of various conditions. To this end, recent spark plugs face severe requirements as to accuracy of an X-directional position of a front end surface (or rather a fee end surface) of a ground electrode that is bent and formed over a front end of a center electrode for the spark gap. This is because the X-directional position or the covering length of the ground electrode extending over the front end of the center electrode is critical to ignition timing of fuel gas injected into present day high performance engine cylinders.
Notably, the covering length of the ground electrode herein means a distance from an X-directional position of the front end surface (i.e., free end surface) of the ground electrode to an X-directional position of the front end surface of the center electrode, as measured in an X direction that is transverse to an axis direction (i.e., Y direction) of a metal shell of a spark plug and normal to a width direction of a fixation portion of the ground electrode formed on the ground electrode.
If the X-directional position of the ground electrode is not fixed at an appropriate position with respect to that of the front end of the center electrode, for instance, if the front end portion of the ground electrode is positioned too far away or too close to the front end of the center electrode, the ignition tinting of the fuel gas by spark discharge of the spark plug in the engine cylinder is delayed or hastened, resulting in reduced total combustion efficiency of the engine. This is because the covering length of the ground electrode extending over the front end of the center electrode affects an approach of the injected fuel gas to the spark gap or aggravates propagation of the ignited gas through the engine cylinder, even if the spark occurs properly at the spark gap formed between the ground and center electrodes of the spark plug. Therefore, since constant-ignition timing is required to a high degree in present day high performance internal combustion engines, the covering length or X directional position of the ground electrode must be fixed or controlled with very small deviation as possible deviating from a predetermined length thereof in manufacturing high performance spark plugs.
In conventional methods and apparatus for manufacturing spark plugs, it has been difficult to fix the X-directional position of the front end surface of the ground electrode at a constant predetermined position with respect to the front end of the center electrode. For example, in the manufacturing method and apparatus disclosed in Japanese Patent Laid-Open No. 121143/1999 (page 10, FIG. 1), the ground electrode is pressed and bent for a spark gap without regulating the X-directional position of the front end surface of the ground electrode with respect to the front end of the center electrode.
On the other hand, in the manufacturing method and apparatus disclosed in EP 0540159 A1 (FIG. 4), the ground electrode formed on the metal shell is pressed and bent while the X-directional position of the front end surface of the ground electrode is regulated. However, because the insulator, center electrode inserted therein and the metal shell holding the insulator and having the pre-bent ground electrode are assembled afterwards, it is difficult to keep or re-fix a constant X-directional positional relationship between the front end of the center electrode and the front end of the ground electrode in the assembled spark plug. This is because a large dimensional deviation is generated during the assembly of these components into the spark plug.